The present invention relates to a programmable transducer device, and in particular to a programmable transducer device that receives a control signal that is superimposed onto the transducer device output signal.
A signal source in the transducer device may be a sensor for magnetic fields, pressure, temperature of the like, which is connected through an output circuit to the transducer output. The output signal (digital or analog) may be conducted via a signal line (e.g., a long signal line) to a receiver (e.g., a control unit). For certain applications, the transducer device may receive commands from the receiver. In this connection one can think of triggering a test operation for the transducer device or of switching in a signal source if, for example, a sensor for temperature and magnetic fields is present in the transducer device. When a sensor is present in the transducer device, it is often desirable to adjust the measurement range of the sensor or to compensate for an offset, which can have internal or external causes. Sometimes auxiliary circuits in the programmable transducer device must also be controlled (e.g., the frequency of a clock pulse generator or the current or voltage at the transducer output). This list is not exhaustive, but is only intended to show, by a few examples, that in many cases it is desirable that the transducer device can be programmed from the receiver.
If a three-legged housing is used for the programmable transducer device, one of the legs is generally permanently at ground potential, and thus only the supply connection and the transducer output are available for programming via the housing connections.
German patent application DE 44 22 867 A1 discloses using the supply connection for programming by modulating the magnitude of the supply voltage. This manner of transmitting a control voltage is often called operating voltage modulation. However, a protective circuit for the operating voltage input is often present, and thus the operating voltage can be varied only within a narrow range. This makes it impossible to input control signals, which must differ clearly from the operating voltage.
It is also known, similar to an input/output connection, that the transducer output can be used both for transmitting signals and for receiving signals. Such a bi-directional data exchange over one input/output data interface requires separate operating states for transmission and reception. However, setting such an operating state can be critical if set by an external signal, which is conducted to the transducer output. In particular, a proper noise signal at the transducer output may erroneously trigger its receiving state, in which the transducer device would then persist. The transducer device thus has been, as it were, shut off by the noise signal, and must first be turned on again.
Therefore, there is a need for a technique for making control signal information available to a programmable transducer device.
An object of the present invention is to provide a programmable transducer device that is actuated by control signals conducted through the transducer output, and which does not need to be switched over to a special receiving state.
Briefly, according to an aspect of the present invention, a transducer device receives an external control signal that is superposed on a transducer output signal line, and a detector circuit within the transducer device detects the control signal. An advantage of this arrangement is that the transducer device is not switched over to receive an external control signal, and consequently cannot be locked in the wrong operating mode. The control signal is detected in the detector circuit by a comparator device that compares the signal superposed on the transducer output line against a reference signal generated by a reference signal generation circuit.
In principle, it makes sense for the reference signal generating circuit to simulate the impedance of the transducer output stage. This includes suitable impedance simulation of a load, for example a resistor, connected to the transducer output. The impedance simulation circuit of the transducer output stage is sensibly driven by the same signal by which the actual transducer output stage is also driven. It is thus possible to obtain, as the reference signal, an image of the transducer output signal under nominal operating conditions. A control signal that acts on the transducer output leads to a deviation from nominal operating conditions.
The comparator device detects the control signal by detecting a clear deviation from the reference signal. If the transducer output signal is an analog voltage signal that can assume arbitrary values, then under normal operating conditions a current will flow in a resistive load, which can be simulated in the reference signal generation circuit. If a load current for the transducer output is known, given the nominal operating range of the transducer, a change of the load current can be recognized as the consequence of a change of the load on the transducer output. Instead, the impedance simulation circuit in the reference signal generation circuit simulates only the nominal load current. Clear deviations in the two signal levels then indicate a control signal, which can be detected in the detector circuit and can be processed in the transducer device.
If the transducer output provides a digital signal as its output signal, a simulation circuit of the output stage can also be used to form the reference signal. However, other reference signal sources (e.g., voltage sources for constant voltages) may also be used. When using voltage sources, the reference signal generation circuit suitably generates an associated comparison voltage as the reference signal, as a function of the particular logical state of the transducer output signal. If this comparison voltage is clearly exceeded above or below, this then indicates the presence of a control signal.
The control signals may be transmitted to the transducer device more reliably if the control signal includes an encoded data sequence, which is associated with a certain instruction in a decoder following the detector circuit. Data words can also be conducted to the transducer device as control signals by the data sequences. For example, these may be equalization data for the signal source, which are written into a memory coupled to the signal source. Suitable codes for safe transmission of individual control signals or of entire data words are also such codes by which the logical state xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d is not transmitted as a static bit value but rather by a change of the logical state within a certain time interval. This criteria would be the direction of the particular change or the presence or absence of such a change. Whether another change occurs outside this time interval is irrelevant. Such changes are easily detectable and make possible an equalized average pulse-pause ratio as well as transmission of a basic clock pulse for function control. Such codes are also known by the general term xe2x80x9cbiphase codesxe2x80x9d. These codes are also suitable for the asynchronous transmission of data, such that the original data sequence can easily be reconstructed again in the decoder by scanning.
Advantageously, providing a programmable transducer device that is actuated by control signals conducted through the transducer output and does not need to be switched over to a special receiving state, ensures the transmission of transducer output signals is not impaired and no additional signal path is required for programming.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.